A double lock-stitch sewing machine is provided with a bobbin received in a bobbin housing and carrying the thread forming the gripper thread of a double lock stitch. This bobbin tends to overrun, upon excessive thread traction, with sudden starting of the sewing machine or when the pull upon the gripper thread is excessive. Such excessive thread traction can result from the actuation of the thread catcher of a thread-cutting device.
As a result of overrunning of the bobbin, the gripper thread can become tangled and can result, upon further operation of the sewing machine, in nonuniformities of the stitched seam, tearing of the thread and the like.
It has been proposed to avoid the overrunning of the gripper-thread bobbin by providing a bobbin brake which can consist of a thin sheet metal spring forming a brake disk which can be placed around the bobbin-carrying pin and can be disposed between the gripper-thread bobbin and a wall of the bobbin housing. This brake disk presses continuously against the gripper-thread bobbin with a light pressure and resiliently seats against the aforementioned wall of the bobbin housing or socket.
In another system, see for example German Printed Application (Offenlegungsschrift) No. 1,816,564, the gripper thread itself actuates a bobbin brake when thread traction of high intensity is applied. In this case the high traction applied to the thread actuates a brake element applying pressure to the bobbin.
The first-mentioned braking device, which acts continuously against the gripper-thread bobbin in the form of a brake disk, is not satisfactory since it cannot, in all cases, prevent an overrunning of the bobbin when a sudden relatively large thread traction develops, e.g. upon actuation of the thread catcher of a thread-cutting device. If the braking force applied by the brake disk is increased to permit it to successfully prevent overrunning of the bobbin in such cases, an impermissibly large tension is continuously applied to the lower thread of the double lock-stitch seam and adversely effects the formation thereof.
In systems in which the lower thread or under thread of the double lock-stitch seam actuates the bobbin brake built into the bobbin housing, it has been found that elastic threads which have increasingly been of interest in recent times cannot be effectively used since a reliable triggering of the braking mechanism cannot be effected with such yieldable or extensible threads.
It has also been proposed heretofore to provide a controlled braking device in which a brake rod or plunger mounted externally of the bobbin housing is displaced laterally into this housing through an opening to engage the bobbin received therein and brake this bobbin by direct contact. This brake member is generally connected via a lever arrangement with a thread-cutting device of conventional construction and is actuated upon operation of the thread catcher to brake the gripper-thread spool and prevent overrunning of the bobbin by reason of the resulting thread traction.
This known bobbin brake is, however, suitable only for horizontal-shaft grippers, because the vertical-shaft grippers disposed between the stitching plate and the bobbin housing leave little room for providing a brake rod or pin adapted to penetrate into the bobbin housing. A further disadvantage of the latter bobbin brake is that it requires a relatively large actuating stroke to bring the brake rod or plunger into play because considerable space must be provided in the region of the bobbin housing to enable replacement of the gripper-thread bobbin.